The electrochemical oxidation of ammonia and hydrazine has been studied for many years. Ammonia oxidation is important in the fabrication of electrochemical sensors for water and air analyses. In addition, ammonia is a common water pollutant in industrial wastewaters and in continental waters. Therefore, the development of an electrochemical method to convert ammonia into nitrogen would open up new possibilities in environmental electrochemistry. Likewise, hydrazine is important in numerous industrial applications, including metal plating and protection against corrosion to control concentrations of dissolved oxygen. It is also used in various rocket fuels and as a component in explosives. Hydrazine is highly toxic and its electrochemical detection is also of significant interest. Finally, the most recent developments in low-temperature fuel cell technology have shown nitrogen hydrides, ammonia, and hydrazine to be suitable candidates in the race for commercial, high-performance, portable fuel cells.
Several studies have been devoted to the electro-oxidation of ammonia and hydrazine on polycrystalline substrates. More recently, studies of ammonia and hydrazine oxidation on single-crystal metal surfaces have been reported, providing evidence that the electro-oxidation process of these small nitrogen hydride molecules is structure-sensitive. For example, in acidified solution, it was found that hydrazine adsorbs more readily on Pt (100) steps than on Pt (111) terraces. While it was shown that the basal planes of platinum and rhodium are much more active for hydrazine oxidation than the corresponding gold surfaces, it was also shown that the (100) plane was one of the most active planes of all three metals. In alkaline media, the electrocatalytic activity of basal planes increases in the order Pt(110)>Pt(100)>Pt(111). Likewise, in alkaline media, the electro-oxidation of ammonia on Pt occurs almost exclusively on surface sites with (100) symmetry.
Therefore, from a practical viewpoint, it would be highly desirable to prepare Pt electrodes that could exhibit both a high electrochemically active surface area and a preferentially-oriented {100} surface structure.
In the early 1970s, an electrochemical method was presented to obtain Pt electrode surfaces with preferred orientations from bulk polycrystalline platinum. The method is based on the use of repetitive potential sweeps at high frequency under carefully-selected potential perturbation conditions. Under the right conditions, the formation of preferentially-oriented {100} surfaces was achieved, but the roughness factor was low and does not exceed R=3. Under these conditions, while the intrinsic electrocatalytic activity (expressed as current per Pt surface atom) for the electro-oxidation of nitrogen hydrides might be high, the overall electrocatalytic activity (expressed as current per geometric surface area) will remain low as a result of the low roughness factor.
Several groups have focused on the use of preferentially-oriented {100} platinum particles to combine both a high intrinsic electrocatalytic activity and high electrochemically active surface area. These particles consist of Pt cubic nanoparticles synthesized in the form of colloidal platinum, using a capping agent (sodium polyacrylate) and hydrogen gas as a reducing agent. According to high-resolution transmission electron microscopy, these nanoparticles show flat surfaces with {100} facets, and the distances between the adjacent lattice fringes is the interplanar distance of Pt {200}. Pt nanoparticles prepared using the same method elsewhere have been shown to exhibit characteristic hydrogen adsorption/desorption peaks, CO-stripping peaks, as well as the characteristic response of irreversibly-adsorbed germanium on (100) sites of platinum. These oriented nanoparticles show higher current densities for the electro-oxidation of ammonia in alkaline media than polycrystalline Pt nanoparticles.
Colloidal methods using organic ligand stabilizers are one of the most commonly used methods to make shape-controlled particles. However, the organic ligand shells can be difficult to remove. Various methods have been devised for cleaning the nanoparticles, such as heating in different atmospheres or submitting the nanoparticle to electrochemical decontamination by surface oxidation. However, these methods could produce a change in the surface structure. It was shown that modification of the surface structure may be limited if electrochemical decontamination is performed under the right conditions, and residual surfactant molecules were still present, causing an incomplete deposition of the Pt nanoparticles (floating) on the surface of the substrate. From a more pragmatic point of view, the preparation of an electrode from an assembly of such nanoparticles is also challenging.
It was shown recently that electrodeposition might be used to prepare metallic particles of various shapes. For example, it was demonstrated that granular Cr nanoparticles or hexagonal microrods could be obtained depending on the deposition conditions, and the synthesis of tetrahexahedral Pd nanocrystals with high Miller Index facets was demonstrated using a pulse electrodeposition method. Likewise, it was shown recently that Pt nanowire with preferentially-oriented {100} surfaces could be prepared through template-assisted deposition, using an anodic aluminum oxide (AAO) membrane.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.